code: def mask_token(self):
      return self.tokenizer.mask_token
  
file_path: OpenPrompt/openprompt/plms/mlm.py
imports: from transformers.models.auto.tokenization_auto import tokenizer_class_from_name
  from openprompt.plms.utils import TokenizerWrapper
  from typing import List, Dict
  from collections import defaultdict
label_desc: Use image node_id 1 for calling the MLMTokenizerWrapper obj's underlying member method code with example usage: obj.mask_token() and returns: self